[I] Digital filters can broadly be classified into finite impulse response (FIR) filters that have a finite impulse response and infinite impulse response (IIR) filters that have an infinitely continuing impulse response. Examples of the IIR filters include, but are not limited to, filters created by simply dividing a frequency range into 1 and 0, Butterworth filters, Gaussian filters using Gaussian functions, and Gabor filters (see Non Patent Literature 4).
The FIR filters have more advantages than the IIR filters in various respects, such as a perfect reconstruction property, fewer errors, and structural stability. In order to design the FIR filters, a certain kind of truncation has often conventionally been performed by applying, for example, a window function technique or a frequency sampling technique to infinitely continuing functions.
There is a technique of using an FIR wavelet as an FIR filter created without performing the truncation. For example, Non Patent Literature 5 discloses that image processing is performed to extract high-frequency components from an original image by summing all subband signals in detail parts up to a certain level, using a wavelet.
[II] It is conventionally reported that an illusion occurs in a certain character string in which the character string appears tilted in a direction different from a character array direction (hereinafter called “letter-row tilt illusion”).
Causes of the occurrence of the letter-row tilt illusion are analyzed using, for example, a wavelet analysis in Non Patent Literature 1 and 2, and using a psychological approach in Non Patent Literature 3. In Patent Literature 1, a technology has been developed to edit fonts so that the letter-row tilt illusion does not occur.